He was in the water during the daytime, isn't that water pretty warm,
Unless the water is around 37C or 99F, which only happens rarely in parts of the Indian Ocean, it's going to lead to substantial heat loss. The more below body temperature the water is, the more rapid that loss will be. (Of course, if it's much above that temperature, it'll lead to overheating.) At a core temperature of around 32C or 90F people will start having atrial arrhythmias and hypotension. At around 28C or 83F, people tend to die of a range of things including ventricular dysrhythmia/asystole pulmonary edema. The rate of temperature decrease even for a constant water temperature depends on numerous variables, but one mathematical model predicts survival of around 30 hours in 23C/83F water. That doesn't take into account any other environmental risks such as wave action, sun exposure, and dehydration.
The sun doesn't contribute enough heat to overcome the massive heat transfer to an actual ocean. If anything, it's likely just to add sunburn injury (I've seen people require skin grafts after a long summer's day in the sun, for example when passed out drunk on the beach).
There's also wave action to consider. Even pretty gentle waves will beat someone up pretty badly. I once had a shrimper who fell off a boat in the Gulf during very calm weather. A couple of days later when he was found he was black-and-blue with substantial rhabdomyolysis (skeletal muscle breakdown) and myoglobinemia (buildup of myoglobin from all those ruptured muscle cells) that led to a very high risk of kidney failure.
Don't forget that hardly anyone falls off a boat into saltwater with a ready source of drinking water. It doesn't take long to become dehydrated.
True. People can easily die of exposure after floating in seemingly-warm, seemingly-calm water for that long.
This wasn't news because someone fell (jumped?) off a cruise ship, which seems to happen fairly often. It was news because they found a body (which doesn't seem to happen often). It was big news because they found a live body, which seems practically never to happen.
